When a fish school is moving, the frequency of an ultrasonic signal reflected by the school of fish is shifted in frequency (Doppler frequency shift) due to the Doppler effect. Due to detection of the Doppler frequency shift ultrasonic echoes from a moving fish school can be distinguished from ultrasonic echoes reflected by a fixed target such as the seabed. Also, a fast or slow speed or the like of the moving fish school can be detected.
A prior art underwater detection system has been provided which detects a Doppler frequency shift (refer to, for example Japanese Unexamined Patent Application Publication No. 29975/1982).
However, the prior art apparatus was merely capable of detecting Doppler frequency shifts of ultrasonic signals coming from one specific direction, but was not capable of detecting Doppler frequency shifts of ultrasonic signals coming from the whole range of directions. Thus, information such as a fast or slow speed or the like of moving fish schools in a wide range of directions can not be obtained in a short time duration.